632 



ZOOLOGY 



divided into eight chambers, situated in the abdomen ; the vessels 

 themselves are not highly developed. The Insecta are, almost 

 without exception, air-breathers, and the organs of respiration 



take the form of branching 

 tubes, the tracheae, by means 

 of which air is conveyed 

 to all parts of the body. 

 The nervous system and 

 sense-organs are highly de- 

 veloped. The excretory organs 

 are a number of blind tubes, 

 the Malpighian tubes, ap- 

 pended to the intestine. The 

 sexes are separate ; develop- 

 ment is sometimes direct, 

 more usually complicated by a 

 metamorphosis. 



Order 1. — Aptera (Collem- 

 bola and Thysanura). 



Insecta in which the wings 

 are absent, and the surface is 

 covered either with scales or 

 hairs. Eyes are sometimes 

 absent ; sometimes there are groups of ocelli ; sometimes com- 

 pound eyes. The segments of the thorax are not fused. Some 

 progress by running, others by springing movements effected by 

 a special springing apparatus on the ab- 

 domen. Some have elongated, many -jointed 

 filaments or cerci at the extremity of the 

 abdomen. Development is direct. 



This order includes the Spring- tails 

 (Podura, Fig. 506), and "Silver-fish" 

 (Lepisma, Fig. 505). 



Order 2. — Orthoptera. 



Insects in which there are two pairs of 

 wings, of which, in most cases, the anterior 

 pair are hard and tough, and the posterior 

 pair delicate and transparent. The parts of 

 the mouth are masticatory. The prothorax 

 is not united with the other segments of 



the thorax. Development is direct, or there is a gradual and 

 incomplete metamorphosis. 



Fit;, .in... Lepisma. (After Guerin and 

 Pcrcheron.) 



Fia. 500. —Podura. (After 

 Guerin and Percheron.) 



